<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <title>8 模板语法-指令</title>
    <script src="js/vue.global.js"></script>
</head>
<body>
<div id = "app">
    千米 : <input type = "text" v-model = "kilometers">
    米 : <input type = "text" v-model = "meters">
</div>
<p id="info"></p>
<script>
    const app = {
        data() {
            return {
                kilometers : 0,
                meters:0
            }
        },
        watch : {
            kilometers:function(val) {
                this.kilometers = val;
                this.meters = this.kilometers * 1000
            },
            meters : function (val) {
                this.kilometers = val/ 1000;
                this.meters = val;
            }
        }
    }
    vm = Vue.createApp(app).mount('#app')
    vm.$watch('kilometers', function (newValue, oldValue) {
        // 这个回调将在 vm.kilometers 改变后调用
        document.getElementById ("info").innerHTML = "修改前值为: " + oldValue + "，修改后值为: " + newValue;
    })
</script>
</body>
</html>